The field of the invention relates generally to permanent magnet assemblies for use in electric machines, and more particularly, to permanent magnet assemblies that include an encapsulating member and methods of manufacturing the same.
Some known electric machines, e.g., those used with electric utility class wind generators and other applications, include large diameter rotors (i.e., a diameter of two feet or greater). One example of such an electric machine is a 2-10 megawatt, medium speed (e.g., 100-400 rpm) permanent magnet generator for wind turbines. Large diameter rotors generally include a plurality of permanent magnet assemblies. At least some known permanent magnet assemblies are formed from multiple magnetic blocks that are secured to the rotor. However, known magnetic blocks, which are typically made from Neodymium Iron Boron (NdFeB), may not be adequately protected from the environment, and as such, such magnets may be vulnerable to corrosion. Corroded magnetic blocks tend to flake, crack, crumble, or otherwise degrade. As a result, corrosion of the magnetic blocks may significantly reduce the reliability and life of the electric machine.
In some known machines, the magnetic blocks are coated with a thin protective layer. For example, the magnetic blocks can be coated with approximately 10-30 micrometers of epoxy or nickel plating. Although such coatings generally facilitate preventing corrosion to the magnets, applying the thin protective layer to the magnetic blocks also adds significant costs to their production. Moreover, the layer may be easily damaged during handling of the magnetic blocks and more particularly, may be damaged during assembly of the electric machine (e.g., attaching the magnetic blocks to the rotor). The damaged portions of the layer leave the magnetic blocks susceptible to corrosion. Thus, the thin protective layers that are sometimes used, may be inadequate to protect the magnetic blocks from corrosion.
In small diameter machines (i.e., machines having a diameter less than two feet), a sealed metallic retaining can or ring is often used to secure the magnetic blocks in position, as well as to provide additional corrosion protection. In large diameter machines, retaining cans and rings are not feasible as the cans and rings may be easily damaged during their installation thereby making them difficult to install on large diameter machines. Within at least some small diameter machines, the magnetic blocks are secured directly to the rotor using fiberglass banding. However, such banding generally is not used with large diameter rotors because it takes a long time for the fiberglass to cure. In addition, the ovens that must be used to cure such a rotor are large and are expensive to construct and maintain. In addition, such ovens require large amount of energy. Moreover, the fiberglass does not prevent circumferential movement of the magnetic blocks with respect to the rotor.
Within at least some large diameter machines, the magnetic blocks are covered with plastic across all of their outwardly-facing surfaces. However, in such embodiments, the surface of the magnetic blocks adapted for face-to-face engagement with the rotor is left uncovered because the plastic may adversely effect the magnetic flux between the magnetic blocks and the rotor. As a result, the uncovered surface is vulnerable to corrosion.